How To See Light Billions Of Years Old

When we look up at the sky, even in the blackness of space, much
of it seems dark and empty.

But in reality, scientists theorized, ancient galaxies should be
lurking out there — even in the parts of the sky in which their
light is too faint to see with the naked eye.

To figure out what's really out there, scientists used the Hubble
to watch a patch of black sky for hours and hours, to detect even
the faintest bit of light emitted from the furthest, faintest
galaxies.

These galaxies are hard to see because they are very far away
from Earth. And the further a light source is, the longer its
light will take to reach Earth and, the more likely that light is
to hit something else along the way and scatter off track, making
the light that reaches Earth fainter.

But it also means that the incredibly faint galaxies we are
looking at are very young, because their light has been traveling
for billions of years at the constant speed of light. By studying
this light, astronomers can measure the distances to these
objects and from that, calculate the ages of far-off galaxies.

This gives us an awesome look at the history of our universe's
evolution over its 13.7 billion-year lifetime. But, to see these
far and faint galaxies, Hubble must spend a long time watching a
dark patch of sky.

1. The Hubble Deep Field

In 1995 — five years after NASA launched Hubble into orbit —
observers at the Space Telescope Science Institute (STSI) trained
the Hubble at a patch of dark sky for 10 consecutive days.

Hubble's main camera took 342 pictures of the extremely small
section of sky — a square measuring a tenth the width of the moon
as viewed from Earth. It's located just above the Big Dipper
constellation:

At the end
of the ten-day stretch, Hubble had collected light from 3,000
objects in this tiny patch of dark sky — an incredibly large
number that no one was expecting. Most of the objects were
galaxies, and more important, was the shapes, colors, and types
of galaxies in the image, called the Hubble
Deep Field. Some of these galaxies included the
youngest, most distant galaxies ever discovered at the time.

"The variety of galaxies we see is amazing," STSI
director Robert
Williams said at the time. "In time these
Hubble data could turn out to be the double helix of galaxy
formation. We are clearly seeing some of the galaxies as they
were more than ten billion years ago, in the process of
formation."

"The past ten days have been an unbelievable
experience," Williams said.

From this Hubble data, astronomers estimate that galaxies today
are producing far fewer stars than they were 8 to 10 billion
years ago. In fact, they seem to have been pumping out 10 times
more stars when they were young, and this star-making has dropped
precipitously as they age.

2. The Hubble Ultra-Deep Field

This image of the Hubble
Ultra Deep Field was released in 2014 and includes additional
data collected from other instruments besides Hubble. The
original Hubble Deep Field was released in
2004.NASA/ESA

To dive deeper into cosmic history, the Hubble team set their
sights on a different portion of the sky — a darker splotch with
few foreground stars that drown out the light from distant
objects for which Hubble was seeking.

Reionization occurred from a few million to one billion years
after the Big Bang. Certain objects present in the universe at
that time, transformed the universe from a foggy haze into the
transparent universe we know today.

Your browser does not support the video tag.
Our Universe VisualizedWhat it might have looked like when the universe went from opaque to transparent.

To better
understand what exactly triggered reionization, Massimo
Stiavelli (mission head for the James Webb Space
Telescope) and his team took 800 exposures over 11.3 days,
during 2003 and 2004. The result is an image containing 10,000
objects (mostly galaxies) — more than three times the number in
the original Hubble Deep Field.

One of the most important discoveries that came out of this image
was the unexpectedly high rate of stars forming during the early
universe — far higher than expected. They discovered this by
measuring how many young stars are present at different epochs.

Young stars emit ultraviolet (UV) light, which astronomers
detect. By measuring the amount of UV light from objects at
different distances, astronomers learned that the early universe
had more young stars than previously thought possible.

This discovery gave astronomers today's leading theory of what
triggered reionization: light from early stars blooming to life.
Energy from these stars could have "ionized" neutral hydrogen
into a state that doesn't block trapped light, making the
universe transparent.

To further investigate this theory, astronomers needed to see
even further back in time.

3. The Hubble eXtreme Deep Field

In 2012, NASA released what remains the deepest view into our
universe: the
Hubble eXtreme Deep Field (XDF). For the image, Hubble's
instruments collected light from over 2000 exposures taken over
23 days of observations, over a period of 10 years.

The XDF contains 5,500 more galaxies than the Hubble Ultra Deep
Field and looks even further back in time: showing galaxies as
they were 13.2 billion years ago — only 500 million years after
the Big Bang.

Some of the galaxies are so faint, they're one ten billionth the
brightness our human eyes can detect. These distant, young
galaxies eventually grew and aged to become mature galaxies like
our home, the Milky Way.

This image is a more detailed look at the same spot of sky as the
Hubble Ultra Deep Field. Except an even smaller section. It's
about 80% the size of the Hubble Ultra Deep Field. The image
below shows just how small this patch of sky, denoted XDF, is:

Astronomers are still not certain if early stars, or something
else, triggered reionization in the early universe, but the hunt
for a solution is far from over.

The next-generation Hubble, the James Webb Space Telescope, is
scheduled to launch in 2018. Its powerful instruments will
look
at the XDF in even more detail. One of the mission's primary science
goals is to better understand the end of what experts call
the "Dark
Ages" of the universe, which includes studying the epoch of
reionization.